The structure, properties and activity directions, catalytic, regulatory and protein modification ability of an enzyme group called transglutaminase (TG-ase – EC. 2.3.2.13) are presented. The basic reaction catalysed by those enzymes is the transfer of primary –ε-aminoacyl residue (e.g. Lys) on the specific place of a γ-amide group of peptide bound Gln; the result is the so-called isopeptide bond formation. However TG-ases can catalyse also many other reactions, e.g. deamidation of a γ-amide group, the nitrosylation and denitrosylation of –SH groups of Cys, the isopeptide bond proteolysis or by means of the complexing the G_h protein (GDP↔GTP) they activate some protein kinases and participate in signal transfer through the membranes (cellular or nuclear). Because of differential functions TG-ases may participate in differently directed posttranslational protein modifications according to multiple mechanisms. In this review the properties and action mechanisms of TG-ases of different origin, including microbial (MTG-ases), are presented. As TG-ases may participate in many metabolic, physiological and regulatory processes bound with protein modification (also those contained in food products), many trials of the application of these enzymes have recently been undertaken to improve in this way the properties of many nutritional products. Therefore much attention is given in this review to present this aspect of TG-ase possibilities. Particularly interesting are those, concerning the cereal, milk, soy and muscle proteins, modification of which would change the structure and properties, such as cross-linking, N-supplementation, gelling, emulsifying, foaming etc.